1. Field of the Invention
This invention relates to a sheet feeding apparatus adapted to send out sheets stacked in a sheet feeding tray, a sheet feeding method, and an image forming apparatus, such as a printer, a copier and a facsimile transfer unit which use the same method.
2. Description of the Related Art
In an image forming apparatus, such as a printer, a copier and a facsimile transfer unit, a sheet feeding apparatus is generally provided so as to output an image formed in an image forming section thereof using an electrophotographic system. In this sheet feeding apparatus, sheets (paper), materials on which information is to be recorded including paper and OHP sheets are stacked in a sheet feeding tray or a sheet feeding cassette, from which the sheets are fed one by one, and sent to the image forming section.
In recent years, the image processing techniques used in this image forming section have progressed, and the formation of a large quantity of images can be carried out practically in a short period of time. In the meantime, it has become necessary to stack sheets in large quantities in a paper feeding tray or a paper feeding cassette, and the increasing of a sheet loading capacity of a paper feeding tray and a paper feeding cassette has been increasingly demanded.
In such an image forming apparatus, a sheet feeding method including lifting sheets after they have been stacked in a paper feeding tray of an increased load capacity by a user (operator), to a position of a predetermined height, and feeding in the same position the sheets one by one from the upper side of the stacked sheets is generally used. To lift the sheets, an elevator tray system in which a sheet-loaded bottom plate is moved up as the bottom plate is kept horizontal, or a lever system in which one end portion of a bottom plate is raised around the other end portion thereof is employed. In these systems, it becomes necessary to set a position (height) of an upper side of stacked sheets to the height of a lower surface of a nudger roll which contacts the upper side of the sheets, and maintain a pressure of the nudger roll against the sheets at a substantially constant level. Namely, in these sheet feeding apparatuses, it becomes necessary that a mechanism for detecting the height of the upper side of the sheets be provided besides a mechanism for detecting the presence or absence of a sheet the provision of which constitutes a prerequisite for the sheet feeding apparatuses.
The sheet feeding apparatuses of the related art include sheet feeding apparatuses disclosed in, for example, Japanese Patent Laid-Open Nos. 213483/1993, 221553/1993 and 172624 and Japanese Utility Model Publication No. 30753/1994. Japanese Patent No. 213483/1993 mentioned above, in which a bottom plate is not lifted, discloses the techniques for detecting a residual quantity of sheets by turning a pickup roll in accordance with the lowering of a sheet takeoff position, and displaying the shortage of the sheets before the sheets have run out. Japanese Patent Laid-Open No. 221553/1993 discloses the techniques for detecting the presence or absence of sheets on the basis of the condition of displacement, which occurs when a pivotable cassette is once lifted by a lifting member, of a takeoff unit provided at an upper portion of the cassette. Japanese Utility Model Publication No. 30753/1994 discloses the techniques for using a single mechanism for detecting both the height of a sheet in the uppermost position of stacked sheets and the run-out of the sheets, and thereby simplifying the structure of the detecting mechanism. Japanese Patent Laid-Open No. 172624/1995 discloses the techniques for detecting the absence of sheets and the height of lifted sheets with reference to the position of a lever type arm provided at an upper portion of a paper feeding tray.
However, according to the techniques disclosed in Japanese Patent Laid-Open No. 213483/1993, it is possible to detect a residual quantity of sheets during the transfer of the sheets but difficult to apply the techniques to the detection of a residual quantity of sheets in a bottom plate lifting type sheet feeding apparatus, and, furthermore, impossible to detect the presence or absence of sheets during an initial operation carried out prior to the lifting of a bottom plate.
According to the techniques disclosed in Japanese Patent Laid-Open No. 221553/1993, the presence or absence of sheets can be detected by a bottom plate lifting mechanism during an initial operation and in the midst of the transfer of the sheets but this detecting operation cannot be carried out until a pivotably provided cassette has been lifted by a lifting member, so that it takes much time before the detection of the absence of the sheets is started. Especially, in the case of a sheet feeding apparatus loaded with a large quantity of sheets, it takes an extremely long period of time until an initial detection of the absence of sheets is carried out, and such a long period of wasteful time poses a problem which can constitute a defect of the commodity specifications.
The techniques disclosed in Japanese Utility Model Publication No. 30753/1994 enable a sheet detecting mechanism to be simplified, and are adapted to execute the detection of the height of sheets and the run-out thereof by a single sensor. Therefore, even in a so-called initial operation, in which, for example, a sheet which has got into a jam is removed with a cover then closed, it is necessary to lower a bottom plate by a predetermined height after it has once been lifted, so that the time carrying out the detection of the presence or absence of the sheets during an initial sheet feeding operation further increases. When a tray of a certain construction is used, it moves wildly while it is lifted during an initial operation, to cause the possibility of breakage of the apparatus to arise.
The techniques disclosed in Japanese Patent Laid-Open No. 172624/1995 are very advantageous in view of their capability of detecting the absence of sheets and the height thereof in a lifted state by a simple structure. However, according to these techniques, the absence of the sheets cannot be detected until a bottom plate has once been lifted. Therefore, just as in the techniques disclosed in each of the above-mentioned publications, a delay in the detection of the absence of sheets during an initial operation in, especially, a large capacity sheet feeding tray poses a problem, and, when a position in which the sheets are set is improper, there is the possibility that a sheet feeding mechanism be destroyed due to the lifting of the sheets.
The present invention has been made so as to solve these technical problems, and provides a sheet feeding apparatus and a sheet feeding method which are capable of detecting the presence or absence of sheets during a so-called initial operation in which a bottom plate has not yet moved up, and which enable a wild movement of a sheet feeding tray during, for example, an upward movement of the bottom plate to be prevented; and an image forming apparatus using the same apparatus and method.
As shown in FIG. 1, the sheet feeding apparatus according to the present invention is characterized by including a tray 1 loaded with sheets P, a sheet nudging member 2 adapted to send out a sheet P from an upper side of the sheets P stacked in the sheet loading tray 1, a first sensor 3 adapted to detect the sheet from an upper side of the sheets P stacked in the sheet loading tray 1, a second sensor 4 adapted to detect the sheets P from a lower side of the same P stacked in the sheet loading tray 1, and a control unit 5 adapted to detect the presence or absence of the sheets P on the basis of an output from the second sensor 4 during an initial operation, and the presence or absence of the sheet P on the basis of an output from the first sensor 3 during and after a sheet feeding operation.
A structural part on which the second sensor 4 is provided is not limited as long as the second sensor 4 is adapted to detect the sheets P from the lower side thereof. This sensor 4 is preferably capable of judging whether the sheets P loaded in a bundle are set correctly or not.
When the sheet loading tray 1 is formed so that it can be set during an initial operation by a user of the sheet feeding apparatus at the time of closing a power source switch of a sheet feeding apparatus body and also after the execution of a sheet stacking operation or an operation for removing a sheet which has got into a jam, for example, when the sheet loading tray 1 is formed so that it can be set in and removed from (withdrawable system) the apparatus body, the operational effect of the second sensor becomes high as long as an upward movement of a bottom plate 6, which will be described later, is made in accordance with the result of the detection of the presence or absence of the sheets P in a case where the sheet loading tray 1 is set after it has once been withdrawn.
This sensor is also effective while an operation for feeding the sheet P by the nudging member 2 is carried out during a sheet feeding operation, and also, while each sheet is fed during an operation for continuously feeding the multiple sheets P. When the feeding of the sheets is done by job, the detection operations of the second sensor carried out between sheets of a job, and also between adjacent jobs are effective.
When the sheet loading tray 1 is formed so as to be characterized by having a bottom plate lifting mechanism 7 adapted to move up after the sheets P have been stacked on the bottom plate 6, the presence or absence of the sheets can be detected by the second sensor 4 before the lifting of the bottom plate 6 by the bottom plate lifting mechanism 7 has been done, so that the lifting of the bottom plate 6 with the sheets not existing thereon can be prevented. Moreover, when a detection operation by the second sensor 4 is not carried out, though the sheets P exist on the bottom plate, the lifting of the bottom plate can be stopped, and this enables a trouble due to a user""s error in the setting of the sheets P to be prevented.
When the second sensor 4 is characterized by being provided on the downstream side with respect to the sheet feeding direction of a position in the vicinity of the sheet nudging member 2, the sensor is preferable in that the sheets P can be lifted on the basis of the result of the detection of the sheets by the same sensor 4 provided on the downstream side of a position in the vicinity of the sheet nudging member 2. Especially, in a case where a system for stopping an upward movement of the bottom plate 6 when the uppermost sheet 2 contacts the sheet nudging member 2 is employed, the wild running of the bottom plate 6 can be prevented even when the user erroneously sets a bundle of sheets P, for example, in an upstream side position with respect to the sheet feeding direction. This can prevent the breakage of peripheral devices of the sheet nudging member 2. Also, it becomes possible to retain the performance of the sheet nudging member 2 of transferring the sheets P. Above all, in an apparatus not provided with an end guide for restricting the position of the sheets on the upstream side with respect to the sheet feeding direction, the condition in which the sheets are not set properly is liable to occur, and the structure according to the invention is specially effective in such an apparatus.
The image forming apparatus according to the present invention is characterized by including an image forming unit adapted to form an image on a sheet P transferred thereto, a sheet loading tray 1 provided therein with a bottom plate 6 which support thereon sheets P to be transferred to the image forming unit, and which is adapted to be vertically moved, a sheet nudging member 2 adapted to feed a sheet P from an upper side of the sheets P stacked on the bottom plate 1 toward the image forming unit, a first sensor 3 provided above the sheet loading tray land adapted to detect the presence or absence of the sheets P, a second sensor 4 provided on the bottom plate 6 in the sheet loading tray 1 and adapted to detect the presence and absence of the sheets P, and a control unit 5 adapted to control a vertical movement of the bottom plate 6 in the sheet loading tray 1, the control unit 5 being adapted to prohibit the bottom plate 6 from moving up when the sheets P are not detected by the second sensor 4 at the time of setting the sheet loading tray 1 on the apparatus.
To make a vertical movement of the bottom plate 6, using an elevator tray system, which is generally applied to a large-capacity tray, for lifting a bottom plate horizontally as well as a system for lifting one end portion of a bottom plate around the other end portion thereof as a fulcrum is conceivable. Although such systems to be employed do not matter, using, especially, the structure according to the present invention enables the time required until the detection of the presence or absence of sheets is started to be reduced, and a great effect to be obtained in an apparatus having a sheet feeding tray on which a large quantity of sheets are stacked, and in which the travel of the bottom plate is large.
Since an upward movement of the bottom plate 6 is prohibited in the mentioned case, the problem of occurrence of an upward movement of the bottom plate 6 which is based on an erroneous recognition given at the sheet loading time, and which leads to the destruction of structures in a sheet feeding section can be solved. From a viewpoint of solving this problem, the second sensor 4 is preferably disposed in a position corresponding to that in which there is a fear of occurrence of the destruction ascribed to an upward movement of the bottom plate of various kinds of structures.
In addition to the provision of such a function of prohibiting an upward movement of the bottom plate 6, providing a function of displaying the absence of sheets on, for example, a control panel of the apparatus, or on a host unit, such as a personal computer when the apparatus is connected thereto via an internet or a line can promote an early response of a user.
When the control unit 5 is formed so that it prohibits an upward movement of the bottom plate 6 during a sheet feeding operation in a case where a sheet P is not detected by the first sensor 3 during the same operation, the sheet P can be detected independently of an operation of the second sensor 4, whereby the detection of the sheet P can be conducted speedily and accurately. Applying such a control unit to, especially, an apparatus loaded with a large quantity of sheets and adapted to feed the sheets by finely lifting a sheet loading tray correspondingly to the lowering of the uppermost sheet surface which is based on the feeding of the sheets is preferable since the absence of sheets can be detected by a simple structure.
The sheet feeding method according to the present invention includes lifting a bottom plate 6 in accordance with the result of detection carried out by a first sensor 3 provided above a sheet loading tray 1 in which sheets P are stacked on the bottom plate 6, and a second sensor 4 provided in a lower portion of the sheet loading tray 1, and feeding the sheets P in order from a top side of the lifted sheets P, characterized in that the bottom plate 6 is lifted during an initial operation on the basis of the result of the detection of the sheets P by the second sensor 4, and during a sheet feeding operation on the basis of the result of the detection by the first sensor 3.
The sheet feeding operation is characterized by being kept carried out irrespective of the result of the detection of sheets P by the second sensor 4. Namely, even when the absence of sheets is detected by the second sensor 4, the operation for feeding the sheets P keeps being carried out as long as the sheets P are detected by the first sensor 3 during the sheet feeding operation. Owing to such operations, judging the condition in which the sheets P are present in the sheet loading tray 1 but merely floated from a lower portion thereof because of the sheet P held by the sheet separating unit 8 as a no-paper condition, and interrupting the feeding of the sheets can be prevented.
The effects of the above-mentioned technical steps will now be described.
In an initial operation in which a user stacks the sheets P correctly on the bottom plate 6 in the sheet loading tray 1 and sets the resultant sheet loading tray 1 on the image forming apparatus, the sheets P are detected by the second sensor 4, and the control unit 5 in the image forming apparatus receives an output from the sensor 4 and recognizes that the sheets P exist on the sheet loading tray 1. The control unit 5 operates on the basis of this recognition the bottom plate lifting mechanism 7 to lift the bottom plate 6 and display such recognized information on a display, such as a control panel provided in the image forming apparatus.
When a sensor (not shown) recognizes that the bottom plate 6 moves up to a position in which the uppermost surface of the stacked sheets P comes into contact at a predetermined pressure with the sheet nudging member 2, a driving power source (not shown) receives a sheet feeding signal from the control unit 5 and drives the sheet nudging member 2 and sheet separating unit provided on the downstream side thereof, the sheets P stacked in the sheet loading tray 1 being taken up from the upper side thereof by the sheet nudging member 2 and sent out. The resultant sheets P are separated one by one by the sheet separating unit 8, and transferred in order toward an image forming unit provided on the downstream side thereof.
These predetermined number of sheets are transferred to the image forming unit, the uppermost surface of the sheets in the tray lowers, so that a contact pressure of the sheets P against the sheet nudging member 2 gradually decreases. Consequently, the transferring of the stacked sheets P becomes difficult if the mentioned contact pressure is left decreasing. Therefore, when the lowering of the upper surface of the sheets is recognized by a sensor (not shown) with the presence of the sheets P recognized by the first sensor 3, the bottom plate 6 moves up by an operation of the bottom plate lifting mechanism 7 to cause the upper surface of the sheets to be brought into contact at a predetermined pressure with the sheet nudging member 2, whereby the transfer of the sheets P is rendered possible.
When the sheets P are stacked by, for example, a user in an initial operation on a downstream side portion of the interior of the sheet loading tray 1, i.e., on such a rear portion thereof that does not permit the sheets to be transferred by the nudging member 2, a no-sheet condition is recognized by the second sensor 4, and the control unit 5 does not operate the bottom plate lifting mechanism 7 with xe2x80x9cNO SHEETxe2x80x9d indicated on a display (not shown). The second sensor 4 is disposed in a position close to the sheet nudging member 2 or on the downstream side thereof, and capable of preventing the destruction, which is ascribed to a wild upward movement of the bottom plate 6, of a sheet transfer mechanism including the sheet nudging member 2.
Even in a case where multiple sheets P are on standby in a held condition at, for example, the sheet separating unit 8 in the midst of a sheet transfer operation, the second sensor 4 detects the condition as xe2x80x9cNO SHEETxe2x80x9d condition. In this case, the sheets P still remain, and it is not preferable to carry out a no-sheet-time operation. Therefore, in the midst of a sheet transfer operation, the control unit 5 carries out a no-sheet-time operation by not using the no-sheet information obtained by the second sensor 4 but using the information from the first sensor 3 which detects the presence or absence of sheets from a position above the sheets P.